I upgraded from 11.10 to 12.04, now when I try to run Firefox from the icon it pops up this: "Missing Profile. Your Firefox profile cannot be loaded. It may be missed or inaccessible." I already tried to remove Firefox and re-installing it. And the command

sudo chown -R $USER:$USER ~/.mozilla

I've also tried to delete the profile file, or renaming it. I've found some solved problems but they didn't work for me. Maybe I should mention that if I run Firefox from terminal using sudo it starts fine.

Well, I've solved it by the moment. I realize that before I upgraded, Firefox was in the version 20.0.1; after upgrading, Firefox is v21.0.0. I've read that Firefox v21 wasn't stable in Linux yet. And maybe that was the trouble, so I downloaded the version 20.0.1 and (I don't know exactly how to install it) ran the file 'firefox-bin' after uninstall the other version and delete de .mozilla folder. It worked! But now Firefox doesn't have is characteristic icon! Can someone tell me to wich folder should I move the Firefox folder (I mean the one that comes in the .tar file)?
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Luis J. RomeroJun 1 '13 at 19:23

Thanks, I will try it, thought, I solved it with the first answer of this question: askubuntu.com/questions/303228/… I realize that the problem is with Firefox 21. I installed an old version (20.0.1) and it worked. Maybe now I can update to FF 21 and try what you say.
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Luis J. RomeroJun 5 '13 at 2:45

I just encountered this in 14.04 after an attempt to move files from my previous install failed. I am no expert, but this worked:
Edited profiles.ini file in ~/.mozilla/firefox/
commented out (using ; below)

;Path=[this.is.a.hash].default

Created a new path line so that the hash matched the [this.is.a.hash].default folder name in ~/.mozilla/firefox/

Maybe I could have renamed the folder to match the name in profiles.ini, but easier to trace back the problem knowing I could delete the new and uncomment the old Path= from within profiles.ini.

Fortunately for me, I installed 14.04 on a new SSD and kept the old 12.x release. Now I'll try what I should have done in the first place - a proper Firefox profile backup/move from the old installation.